1. Field of the Invention
The present invention relates to a fixing device and a fixing roller for fixing a non-fixed image of an image visualizing agent (toner) on a recording medium, which image is formed and carried on a recording medium such as a transfer medium, a photosensitive paper sheet, an electrostatic recording paper sheet, or the like by a transfer method (indirect method) or a direct method in an image forming means section which adopts an appropriate image formation principle or process such as electrophotography, electrostatic recording, magnetic recording, or the like in an image forming apparatus such as a copying machine, a recording apparatus, a printer, a micro-reader printer, or the like.
2. Related Background Art
As a conventional fixing device for a general image forming apparatus such as an electrophotographic copying machine, devices adopting various fixing methods have been proposed and are practiced. Of these devices, a device of a roller fixing type, in particular a heating roller fixing device (heat roll fixing method) in which at least one of a pair of rollers is a heating roller heated by a heat source is popularly used.
Of the pair of fixing rollers, a roller contacting the image carrier surface of a recording medium will be referred to as a fixing roller or heating roller hereinafter, and the other roller will be referred to as a compression roller hereinafter.
As the fixing roller, the following two rollers are known:
1 a so-called "hard roller" prepared by coating a fluorine resin such as a PFA (a copolymer resin of tetrafluoroethylene and perfluoroalkylvinylether), a PTFE (a polytetrafluoroethylene resin), or the like onto the outer surface of a core metal such as aluminum, iron, or the like; and PA1 2 a so-called "soft roller" prepared by stacking an elastic rubber layer of, e.g., silicone rubber, fluorine rubber, or the like, on the outer surface of a core metal.
Since the hard roller has high mold release characteristics, it has merits such as high anti-offset characteristics, a high heat resistance, a high durability, and the like.
Since the soft roller has elasticity, it does not excessively press a visualized image (toner image) on a recording medium, and does not blur the image. Therefore, a fixed image has high image quality. Also, since the roller surface finely elastically deforms at the pressing nip portion, the contact area with a toner image on the recording medium can be widened. Therefore, the soft roller has high fixing characteristics.
The compression roller is required to have proper elasticity for attaching a toner image onto the recording medium, and a roller prepared by stacking a layer of silicon rubber or fluorine rubber on the outer surface of a core metal such as aluminum, iron, or the like, a roller prepared by coating the outer surface of the former roller with a PFA tube, or the like is used.
However, as for the fixing roller, the merits of each of the above-mentioned hard and soft rollers are also the demerits of the other.
More specifically, since the hard roller does not have any elasticity, it suffers problems associated with deterioration of image quality caused by excessive pressing of a toner image on a recording medium, and poor fixing characteristics due to a small contact area with the toner image.
Since the soft roller is used in a high temperature range from 180.degree. C. to 200.degree. C., its rubber coating layer is degraded, and the soft roller suffers a problem associated with poor durability.
Also, the compression roller suffers problems associated with a decrease in elasticity, peeling of the PFA tube, and the like caused by thermal degradation of the rubber coating layer.
In order to solve these problems, as a roller material for obtaining a fixing roller which has merits of both the hard and soft rollers, and a compression roller free from the above-mentioned problems, a perfluoroelastomer having rubber elasticity including characteristics such as elasticity, heat resistance, corrosion resistance, high mold release characteristics, and the like, and its improved material (Japanese Laid-Open Patent Application Nos. 50-109280, 59-80457, 59-109546, 62-15212, 3-172311, and the like) have received a lot of attention, and plausibility of their application was examined.
However, when a perfluoroelastomer having rubber elasticity is molded into a fixing roller, or the like, it is difficult to process this material into a complicated shape due to necessity of compression molding in molds, elasticity of the material itself, and the like, as described in Japanese Laid-Open Pat. Application Nos. 57-80040and 3-265768, and the material is not positively utilized in the fixing roller and/or the compression roller of the fixing device.